Global warming due to the emission of CO2 into the atmosphere is a public concern. This, combined with the possible introduction of commercial incentives to limit greenhouse gases, particularly atmospheric carbon, CO2 control technologies are in demand. Electricity and heat generation account for 41% of the total CO2 emissions in 2005, and the demand will be doubled by 2030 according to the World Energy Outlook projects. One method to reduce CO2 emissions by power generation sectors of industry is the use of non-carbon power generation, such as by hydropower and nuclear power generation. However, fossil fuels currently are the most important sources of world electricity and heat generation, providing 70% of the generation. This is particularly so for coal, which supplied 39% of the electricity generation in 2005, and which is sufficiently abundant to be used for the next 130 years at the coal production rates of 2007. This suggests that clean coal technology, which includes the process known as CO2 capture and storage (CCS), is likely to be an important part of the solution to reduce CO2 emissions.
Under a CCS system, CO2 is captured from coal power plants and then transported and stored rather than being emitted to the atmosphere. There are a range of factors affecting the application of CCS and two important ones are the technical maturity and costs. The CO2 capture process is the most expensive component in most CCS systems. Therefore it is critical to develop low-cost CO2 capture technologies to make the commercialization of CCS possible. In fact, in order for the CO2 capture technologies to be widely adopted, they must ideally bring economic gain rather than loss.
One alternative is the development of new systems of power generation with in-situ CO2 capture. In these systems, CO2 capture materials are used under high temperatures. Examples of such systems include the zero emission coal technology proposed by the Zero-Emission Coal Alliance (ZECA) with a claimed efficiency of 68%, and the Japanese Hydrogen Production by Reaction Integrated Novel Gasification process (HyPr-RING) with a claimed plant efficiency of over 53%. In these systems, the CO2 capture material plays a very important role in enhancing the overall efficiency of the systems. When coal or natural gas is gasified (reformed) together with the CO2 capture material, high purity hydrogen is produced while CO2 is captured in the material. Then, the hydrogen, free of CO2, may be used a source of carbon-free energy to generate power. These and other techniques are referred to as advanced zero emission power (AZEP) generation technologies.
Another particularly important use of a CO2 capture material would be in sorption-enhanced steam methane reforming (SE-SMR). In SE-SMR, natural gas is reformed with pressurised steam and oxygen in a reforming reactor to produce synthesis gas (CO+H2). If a CO2 capture material is also added into the reactor, high purity hydrogen will be generated, and the H2 may then be used separately as a source of energy.
A further important application of a CO2 capture material is in CO2 separation from a gas mixture. When the gas mixture is passed through a reactor bed of CO2 capture material, CO2 will be captured by the material while the rest of the mixture will exit the reactor (assuming the other gases in the mixture do not react with the material). Thus, the application of the capture material in post-combustion capture of CO2 in the flue gas of coal-fired power plants is possible. Obviously, other uses for a CO2 capture material can be envisaged.
From a commercial perspective, the CO2 capture material should ideally (a) be sufficiently robust that it can be recycled numerous times without any appreciable loss of performance in terms of its CO2 capture and liberation abilities (known as reversibility), (b) have a fast reaction rate, and (c) have a high capture capacity. Some suitable sorbents for CO2 capture at high temperature have been identified in the art to be Li2ZrO3/Li4SiO4 and CaO. CaO is cheap and widely available which makes it very attractive.
Indeed, CaO has been identified to be thermodynamically one of the most suitable materials for CO2 capture at high temperatures (eg in the range of about 300° C. to about 700° C.) due to its fast reaction rate and high capture capacity at those conditions. Additionally, it can be used either on its own or in combination with other minerals, where CaO is the main component. The reactions involved in the CO2 capture process can be described by equations (1) and (2).CaO+CO2→CaCO3(CO2 capture by CaO-carbonation/adsorption)  (1)CaCO3→CaO and CO2(release of CO2 from CaCO3-calcination/desorption)  (2)
However, the CO2 capture capacity and reversibility of CaO and CaO-based materials reduces after a few carbonation/calcination cycles. This is believed to be due to the CaCO3 undergoing sintering at the high temperature required for calcination. Sintering is a phenomenon where the CaCO3 crystals adhere to each other and results in a decrease in reaction surface area of the sorbent, which results in lower capture capacity for CO2 in the next carbonation cycle.
It would be desirable for the problems with the currently available CO2 capture materials, and particularly CaO systems, to be avoided or at least alleviated or ameliorated.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.